This invention relates to improved methods for producing plastic scintillator material. Simply stated scintillator is a material that converts energy into light. Energy is deposited into the scintillator by penetrating radiation. This energy is then converted into ultra-violet or visible light which can then be detected with the use of a photodetector such as a photomultiplier tube. Generally, incident penetrating radiation includes high energy particles and ionizing radiation such as x-rays, gamma-rays, alpha particles, beta particles, thermal neutrons, etc.
Standard plastic scintillator consists of a polymer matrix typically doped with two fluorescent organic compounds, a primary dopant and a secondary dopant. Solutions of a polymer and an efficient fluorescent dopant can give a system with close-to-unity quantum yield, a high-efficiency scintillator. The term quantum yield is defined as the probability that an excited state in the polymer leads to the emission of a photon by a dopant, which is generally an organic compound. In Forster's theory of nonradiative energy transfer between molecules in solution, the energy transfer is described by a dipole-dipole interaction in which nonradiative energy transfer occurs between the first excited .pi.-singlet state of the solvent and the solute(dopant) in the ground state. The strength of this interaction is a function of the natural fluorescence lifetime of the solvent and the mean separation between the solvent and the solute molecule. Fluorescence is defined as the emission of a photon by a molecular transition from the molecule's first excited singlet state to its ground state. At high dopant concentrations (approximately 1% by weight), this process can dominate the emission or quenching process of the solvent. If the dopant's radiative quantum yield is close to unity, the number of photons emitted per solvent-molecule .pi.-electron excitation can approach one even though the radiative quantum yield of the solvent is small (in the case of polystyrene, for example, 0.07).
Dopants that couple to the primary excitation of the solvent are called primary dopants. The primary dopant serves to raise the photon yield of the solvent plus dopant(s) combination (the number of photons emitted per unit energy deposited in the solvent) and to shift the mean wavelength of the final fluorescence of the scintillator to longer wavelengths. The addition of secondary dopants, in low concentrations (approximately 0.01% to 0.2% by weight), to the binary system of solvent and primary dopant results in the shifting of the fluorescence wavelength further to the red portion of the spectrum. Secondary dopants do not increase the intrinsic photon yield of the scintillator. However, by shifting the fluorescence emission to a longer wavelength, more photons escape from the scintillator since self-absorption by the scintillator is reduced. The term "technical quantum yield" is often applied to scintillators of finite size. In this case, effects of self-absorption are included in the determination of quantum or detected photon yield of the scintillator.
The most commonly used polymer bases in plastic scintillator are polystyrene and poly(vinyltoluene). Monomeric styrene is a liquid which polymerizes on heating to form the solid plastic polystyrene. When doping of the polystyrene or some alternate polymer matrix is required, several methods are available including:
1. dissolving the dopant in molten polystyrene, PA1 2. polymerizing a styrene-dopant solution at a low temperature (50.degree. C.) with a benzoyl peroxide catalyst, PA1 3. polymerizing a styrene-dopant solution at a medium temperature (125-140.degree. C.) over several days without a catalyst, PA1 4. polymerizing a styrene-dopant solution at a high temperature (200.degree. C.) without a catalyst for a period of 12-15 hours. PA1 1. purification of the monomer by vacuum distillation; PA1 2. addition of dopants to monomer; PA1 3. removal of dissolved gasses, notably oxygen; PA1 4. complete polymerization in an inert atmosphere or under vacuum; and PA1 5. careful annealing.
Method 2 was found to impair the transparency and to reduce the efficiency of the scintillator.
The size of the plastic scintillator frequently dictates the production method used. For small samples, the monomer is generally subjected to numerous vacuum distillation processes to remove inhibitors. The monomer is then transferred to a vial containing the dopant. The vial is connected to a vacuum system, and the dissolved gas is removed by repeated freeze-pump-thaw cycles. The evacuated vial is sealed, shaken to insure the complete dissolution of the dopant in the monomer, and then placed in an oil bath at 125.degree. C. for several days. For larger specimens, a generally employed technique is to bubble nitrogen through the monomer to expel the oxygen and then mix in the dopant. The solution is then heated and the polymerization is carried out in a nitrogen atmosphere. For high temperature polymerization, the vacuum-distilled monomer and the dopant are placed in a reaction flask fitted with a reflux condenser and flushed with nitrogen for approximately thirty minutes after which the container is evacuated and sealed. The container is then heated in a bath to over 200.degree. C. for a period of from eight to ten hours.
Typically, the technique for the preparation of plastic scintillators is:
Applicants produce plastic scintillator using two new methods. In the first method, the plastic scintillator material is produced by combining polystyrene pellets, which have been purged with an inert gas, with oil, and then mixing in the dopants. Mineral oil was used with some success; however, significantly better results were achieved when silicone oil was used as a plasticizing agent to coat the polymer with dopant during mixing. The preferred silicone oil is an aromatic-substituted silicone. The pellet-oil-dopant combination is compounded or processed in an inert gas atmosphere, argon or nitrogen, to generate plastic scintillating material that can then be pelletized. These scintillator pellets can subsequently be extruded, injection molded and/or exposed to other plastic molding processes to form a scintillator piece of a particular shape or form. This process can also directly extrude or scintillating fibers, sheet, or film from the melt, thus avoiding the pelletizing of the new scintillating plastic. By purging the initial polymer pellet stock with argon, the water and oxygen are driven out of the pellets. This and the subsequent use of argon or nitrogen in the processing of the mixture eliminate or reduce the need for a vacuum.
In the second method for producing plastic scintillator material, the need for pre-mixing a polymer-pellet oil dopant combination is eliminated by using an inline coloring and compounding extrusion process. In this method the polymer pellets and all additives (dopants, silicone oil if desired, etc.) are metered into the processing device and again a plastic scintillator material is produced. The use of an inert gas purge is also incorporated in this method. Scintillator profiles, sheet, film, fiber can be directly produced by this method or the scintillator material can be pelletized for subsequent plastic forming operations.
Applicants' processes produce a plastic scintillating material which exhibits a very uniform distribution at the molecular level; essentially, a uniform solution of the solutes "dopants" in the solvent "polymer". The uniform distribution at the molecular level is necessary for the scintillation mechanism to function reliably.
Until now, the production of high light yield scintillators was limited to relatively slow methods as described by the prior art; however, applicants' methods produce plastic scintillating material using a continuous process that can operate over a wide range of production rates.
Thus, one object of this invention is to produce a plastic scintillating material with uniform solute-solvent distribution at the molecular level using commercial processes that can accomplish this at high rate (hundreds of pounds per hour) and in a continuous process as opposed to a bulk or batch process.
Another object of this invention is to employ an inert gas blanket before and during extrusion to expel the water and oxygen, and to prevent unwanted interactions with the polymer and the resulting scintillator reducing, thus, the need for a vacuum.
Another object of this invention, in the first method, is the implementation of silicone oil as the plasticizing agent to uniformly and homogeneously disperse the scintillation dopants into the polymer. When mineral oil is used, it tends to discolor the scintillator and consequently reduces the light yield of the scintillator.
Additional advantages, objects and novel features of the invention will become apparent to those skilled in the art upon examination of the following and by practice of the invention.